1. Field of the Invention
The present invention relates to pilot valves widely used as a device for controlling hydraulic implements attached to agricultural, construction and other such machinery. More particularly, the invention relates to a pilot valve comprising:
a valve case; PA1 a spool reciprocable within the valve case between a pilot pressure supplying position for interconnecting a pump port and a supply and exhaust port, and an exhaust position for interconnecting a tank port and the supply and exhaust port; PA1 a push rod movable in a first direction within the valve case to move the spool to the supplying position, the push rod including a cylindrical skirt portion adjacent one end thereof having sliding surfaces for sliding relative to the valve case, and a rod portion adjacent the other end, the skirt portion, the rod portion and the valve case defining a first space, the skirt portion, the spool and the valve case defining a second space, the first space and the second space being opposed to each other across the skirt portion; PA1 a balancing spring interposed between the spool and the skin portion for adjusting a pilot pressure; PA1 a return spring for biasing the push rod in a second direction opposite to the first direction; and PA1 a control member for acting on the rod portion to move the push rod in the first direction against a biasing force of the return spring.
2. Description of the Related Art
Well-known pilot valves of this type include a crosswise controllable valve as disclosed in Japanese Utility Model Publication Kokai No. 3-17386. This pilot valve has coupling ports defined in a bottom surface of a valve case for connection to piping extending to control valves. Further, supply and exhaust valve mechanisms are provided, each of which includes a spool slidable between one of the coupling ports, and a pump pressure chamber and a tank pressure chamber arranged vertically within the case, and a push rod for receiving one end of the spool and transmitting a force to the spool through a balancing spring for adjusting a pilot pressure. With this construction, the push rod is movable in an amount corresponding to an amount of operation of a control lever. The push rod cooperates with the balancing spring to move the spool, thereby generating a corresponding pilot pressure.
Such pilot valves may be classified into two types based on the push rod structure, which are:
1) the type in which, as shown in FIG. 1 of the above publication, the push rod includes a rod portion (referenced 8) for receiving an operating force from the control lever, and a skirt portion (referenced 6) formed separately from the rod portion for receiving a return spring (referenced 7); and
2) the type in which, as shown in FIG. 2 of the above publication, the rod portion and skirt portion are formed integral with each other.
In the former, combined structure, the rod portion and skirt portion may be manufactured as individual components with ease. However, the two portions must be independently slidable relative to the valve case, but their sliding surfaces are short. This results in disadvantages in terms of smooth sliding movement and shaping precision. In the later, integrated structure, although long sliding surfaces are secured, a space is formed between an upper surface of the skirt portion and a cover member forming part of the valve case when the push rod is depressed, and it is necessary to deal with this space. That is, expansion or compression of this space occurring with movement of the push rod produces a resistance to hinder smooth movement of the push rod.